In such systems, flocculant, usually a polymer flocculant, is added or "dosed" to a sludge being dewatered in a plate press system which incorporates a belt press. Polymer is expensive, and the normal tendency of an operator when dewatering is nor progressing sufficiently well is to add more polymer. However, in addition to the expense, this does not necessarily improve the dewatering efficiency. Moreover, it has been found that on optimisation of polymer sludge conditioning variable mixing control has been identified as a desideratum to improve performance of sludge dewatering equipment. If the correct mixing energy is imparted at the right time then dry solids yield would be increased and polymer demand reduced during sludge conditioning using polymer addition.
It is accordingly an object of the invention to seek to provide mixing apparatus which mitigates the aforesaid disadvantages and seeks to optimise an optimum mixing energy over a range of sludge feed rates.
According to one aspect of the invention there is provided mixing apparatus for mixing a flow of sludge comprising means to monitor pressure differential across the apparatus, and means to vary the size of an orifice of the apparatus, dependent on the pressure differential, whereby to maintain a desired differential pressure over a range of feed rates to the apparatus.
The apparatus may comprise a mixing chamber with a plate valve adapted to determine the size of the orifice. This is a relatively simple yet efficient apparatus.
The orifice may comprise an outlet orifice of the chamber. This provides for ready control.
There may be means adapted to vary addition of flocculant to the sludge dependent on the pressure differential. This is a relatively simple way to control polymer dosage, particularly where the means may comprise electronic means adapted to actuate a flocculant dosing element for dosing the sludge with flocculant.
The dosing element may comprise a polymer flocculant dosing pump. This is a relatively straightforward means for adding polymer flocculant to the sludge.
The means to monitor the pressure drop may comprise a pressure sensor at an inlet side of the apparatus and a pressure sensor at the outlet side of the apparatus, the two sensors providing a control signal to the means to vary the size of the orifice. This system can provide for automatic control of the apparatus.
The means to vary the size of the orifice may comprise an electronic actuator operatively connected with the plate valve. This provides for positive control, whether automatic or not.
The plate valve may comprise an inclined plate. This provides for positive control of outlet orifice size.
According to a second aspect of the invention, there is provided a sewage dewatering system, incorporating a mixing apparatus as hereinbefore defined.
The system may suitably be a belt press system.
According to a third aspen of the invention, there is provided a method of treating a sewage sludge prior to dewatering, comprising providing a flow of sludge through a mixing apparatus, measuring a pressure differential across the mixing apparatus, and maintaining a desired differential pressure over a range of flow rates.
Mixing apparatus embodying the invention is hereinafter described, by way of example, with reference to the accompanying drawings.